Method and Apparatus for Chuck Thermal Calibration

ABSTRACT

Wafer temperature is measured as a function of time following removal of a heat source to which the wafer is exposed. During the wafer temperature measurements, a gas is supplied at a substantially constant pressure at an interface between the wafer and a chuck upon which the wafer is supported. A chuck thermal characterization parameter value corresponding to the applied gas pressure is determined from the measured wafer temperature as a function of time. Wafer temperatures are measured for a number of applied gas pressures to generate a set of chuck thermal characterization parameter values as a function of gas pressure. A thermal calibration curve for the chuck is generated from the set of measured chuck thermal characterization parameter values and the corresponding gas pressures. The thermal calibration curve for the chuck can be used to tune the gas pressure to obtain a particular wafer temperature during a fabrication process.

CLAIM OF PRIORITY

This application is a divisional application of U.S. patent applicationSer. No. 11/198,489, filed on Aug. 5, 2005, the disclosure of which isincorporated in its entirety herein by reference.

BACKGROUND

Semiconductor wafer (“wafer”) fabrication often includes exposing awafer to a plasma to allow the reactive constituents of the plasma tomodify the surface of the wafer, e.g., removal material from unprotectedareas of the wafer surface. The resulting wafer characteristicsfollowing the plasma fabrication process are dependent on the processconditions, including the plasma characteristics and wafer temperature.For example, in some plasma processes a critical dimension, i.e.,feature width, on the wafer surface can vary by about one nanometer perdegree Celsius of wafer temperature. It should be appreciated thatdifferences in wafer temperature between otherwise identical waferfabrication processes will result in different wafer surfacecharacteristics. Thus, a drift in process results between differentwafers can be caused by variations in wafer temperature during plasmaprocessing.

A general objective in wafer fabrication is to fabricate each wafer of agiven type in as identical a manner as possible. To meet thiswafer-to-wafer uniformity objective, it is necessary to controlfabrication parameters that influence the resulting wafercharacteristics. Therefore, it is necessary to control the wafertemperature during the plasma fabrication process. Current plasmaprocessing devices for wafer fabrication do not include effective wafertemperature control capabilities. Therefore, a need exists for accuratecontrol of wafer temperature during a plasma fabrication process.

SUMMARY

It should be appreciated that the present invention can be implementedin numerous ways, such as a process, an apparatus, a system, a device ora method. Several inventive embodiments of the present invention aredescribed below.

In one embodiment, a method is disclosed for performing a thermalcalibration of a chuck. The method includes an operation for supportinga wafer on the chuck in exposure to a heat source. The method continueswith an operation for applying a gas at a substantially constantpressure at an interface between the wafer and the chuck. Then, the heatsource is removed. Following removal of the heat source, a temperatureof the wafer as a function of time is measured while maintaining theapplied gas pressure. The method further includes an operation fordetermining a chuck thermal characterization parameter value based onthe measured temperature of the wafer as a function of time. Thedetermined chuck thermal characterization parameter value is directlycorrelated to the applied gas pressure. The aforementioned methodoperations are repeated for a number of different applied gas pressuresto generate a set of chuck thermal characterization parameter values asa function of gas pressure. A correlation is then established betweenthe measured chuck thermal characterization parameter values and thecorresponding gas pressures to generate a thermal calibration curve forthe chuck. It should be appreciated that using the generated thermalcalibration curve for the chuck, a gas pressure can be determined for aparticular chuck thermal characterization parameter value, vice-versa.Additionally, the thermal characterization parameter represented in thethermal calibration curve for the chuck can be directly correlated towafer temperatures during the wafer fabrication process.

In another embodiment, a method for controlling wafer temperature duringa wafer fabrication process is disclosed. The method includes anoperation for determining a target thermal characterization parametervalue for a chuck. It should be appreciated that the chuck is to be usedto hold the wafer during the wafer fabrication process. The methodfurther includes an operation for examining a thermal calibration curvefor the chuck to determine a backside gas pressure value correspondingto the chuck target thermal characterization parameter. It should beunderstood that the backside gas pressure corresponds to a pressure of agas applied at an interface between the wafer and the chuck. The methodfurther includes an operation for setting the backside gas pressure tothe value corresponding to the chuck target thermal characterizationparameter value. It should be appreciated that the backside gas pressureserves to control the wafer temperature during the fabrication process.

In another embodiment, a system is disclosed for providing wafertemperature control during a wafer fabrication process. The systemincludes a chuck defined to hold a wafer in exposure to a plasma. Thechuck includes a number of ports to supply a gas at an interface betweenthe wafer and the chuck. The system also includes a gas controllerdefined to control a pressure of the gas supplied at the interfacebetween the wafer and the chuck. The system further includes a computingdevice for controlling the gas controller. The computing device includesthermal calibration data for the chuck, wherein the thermal calibrationdata specifies a gas pressure setting to be communicated from thecomputing device to the gas controller to maintain a target wafertemperature.

Other aspects and advantages of the invention will become more apparentfrom the following detailed description, taken in conjunction with theaccompanying drawings, illustrating by way of example the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing a generalized representation of aplasma chamber for semiconductor wafer processing, in accordance withone embodiment of the present invention;

FIG. 2 is an illustration showing a vertical cross-section view of thechuck, in accordance with one embodiment of the present invention;

FIG. 3 is an illustration showing transient wafer temperaturemeasurements for a chuck designated “ESC 1” at various backside gaspressures, in accordance with one embodiment of the present invention;

FIG. 4 is an illustration showing exemplary thermal characterizationparameter versus backside gas pressure curves, in accordance with oneembodiment of the present invention;

FIG. 5 is an illustration showing a flowchart of a method for performinga thermal calibration of a chuck, in accordance with one embodiment ofthe present invention; and

FIG. 6 is an illustration showing a flowchart of a method forcontrolling wafer temperature during a wafer fabrication process, inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one skilled in the art that the presentinvention may be practiced without some or all of these specificdetails. In other instances, well known process operations have not beendescribed in detail in order not to unnecessarily obscure the presentinvention.

FIG. 1 is an illustration showing a generalized representation of aplasma chamber 100 for semiconductor wafer (“wafer” hereafter)processing, in accordance with one embodiment of the present invention.The chamber 100 is defined by surrounding walls 101, a top 102, and abottom 104. A chuck 103 is disposed within the chamber 100 to hold thewafer 105 in exposure to a plasma 107 to be generated within the chamber100. In one embodiment, the chuck 103 is defined as an electrostaticchuck (ESC) capable of being electrically charged to attract ions in theplasma 107 toward the wafer 105. In one embodiment, a coil 109 isdefined above the chamber to provide energy for generating the plasma107 within the chamber internal volume.

During operation, a reactant gas flows through the chamber 100 from agas lead-in port (not shown) to a gas exhaust port (not shown). Highfrequency power (i.e., RF power) is then applied from a power supply(not shown) to the coil 109 to cause an RF current to flow through thecoil 109. The RF current flowing through the coil 109 generates anelectromagnetic field about the coil 109. The electromagnetic fieldgenerates an inductive current within the etching chamber 100 internalvolume. The inductive current acts on the reactant gas to generate theplasma 107. During the etching process, the coil 109 performs a functionanalogous to that of a primary coil in a transformer, while the plasma107 performs a function analogous to that of a secondary coil in thetransformer.

The plasma 107 is enveloped by a sheath of non-plasma reactant gas.Therefore, high frequency power (i.e., RF power) is applied from a powersupply through matching circuitry to the chuck 103 to providedirectionality to the plasma 107 such that the plasma 107 is “pulled”down onto the wafer 105 surface to effect the etching process. Theplasma 107 contains various types of radicals in the form of positiveand negative ions. Chemical reactivity of the various types of positiveand negative ions are used to etch the wafer 105.

FIG. 2 is an illustration showing a vertical cross-section view of thechuck 103, in accordance with one embodiment of the present invention.The chuck 103 is defined by multiple material regions including aceramic layer 203, an adhesive layer 205, and an aluminum layer 206. Thechuck 103 also includes a number of cooling channels through which aliquid coolant, e.g., water, flows. The wafer 105 is held on a surface202 of the chuck 103 during plasma processing. In one embodiment,mechanical force is used to hold the wafer 105 on the surface 202. Inanother embodiment, electrical force is used to attract the wafer 105 tothe surface 202 and hold the wafer 105 during processing.

To avoid unnecessarily obscuring the present invention additionalfeatures of the chuck 103 have not been shown in FIG. 2. For example, inaddition to the fluid coolant channels 207, the chuck 103 also includesgas coolant channels defined to direct a gas coolant to regions betweenthe wafer 105 and the surface 202 of the chuck 103. Furthermore, itshould be appreciated that the fluid and gas coolant channels of thechuck can be configured in essentially any arrangement necessary toproduce a desired operational effect. Additionally, the chuck 103 can bedefined to include a number of lifting pins to assist in chucking andde-chucking of the wafer 105. Furthermore, it should be understood thatthe chuck can be electrically connected to a power source throughmatching circuitry to provide the plasma 107 directional bias and/orelectrical wafer clamping force as previously mentioned. Thus, the chuck103 is actually a complex device that includes more components thanexplicitly shown in FIG. 2.

Wafer characteristics following plasma processing are typicallydependent upon a temperature of the wafer during the plasma processing.For example, in some plasma processes a critical dimension, i.e.,feature width, on the wafer can vary by about one nanometer per degreeCelsius of wafer temperature. Additionally, a drift in process resultsbetween different wafers can be caused by variations in wafertemperature during the plasma processing of the different wafers. Ageneral objective in wafer fabrication is to fabricate each wafer of agiven type in as identical a manner as possible. Therefore, to meet thiswafer-to-wafer uniformity objective, it is necessary to controlfabrication parameters that influence the resulting wafercharacteristics. Obviously, the plasma conditions will directly affectthe resulting wafer characteristics. However, as previously mentioned,the wafer temperature during the plasma process will also directlyaffect the resulting wafer characteristics.

In addition to holding the wafer 105 and serving as a mechanism fordirectionally biasing the plasma 107, the chuck 103 also acts as theprimary mechanism for controlling a temperature of the wafer 105 duringprocessing. During operation, heat flux emanating from the plasma 107 isdirected toward the wafer 105. Thermal energy absorbed by the wafer 105,is conducted from the wafer through the various regions of the chuck 103to the fluid coolant within the coolant channels 207. Thus, the fluidcoolant within the coolant channels 207 serves as a primary heat sink.Therefore, it should be appreciated that a temperature of the wafer 105during processing is not only dependent upon the heat flux emanatingfrom the plasma 107, but is also dependent upon the thermalcharacteristics of the chuck 103, i.e., how heat is conducted from thewafer 105 to the heat sink.

With respect to the chuck 103 representation of FIG. 2, heat isconducted from the wafer 105 to the fluid within the coolant channels207, via conduction through the various chuck 103 materialstherebetween. At least two paths exist for thermal conduction betweenthe wafer 105 and the ceramic layer 203. In one path, heat is conducteddirectly from the wafer 105 to the ceramic layer 203 via physicalcontact. In another path, heat is conducted through gas present withingaps 201 between the wafer 105 and the ceramic layer 203. The gaps 201result from the random microscopic surface roughness of the chuck 103and/or engineered surface topography such as bumps. It should beappreciated that a size of the gaps 201 in FIG. 2 is exaggerated forease of discussion. The conduction through the gas within the gaps 201is dependent upon the gas pressure and the gap 201 characteristics. Inone embodiment, gas, e.g., helium, is provided to the gaps 201 throughports on the surface 202. The gas pressure can be controlled to adjustthe amount of thermal conduction through the gas-filled gaps 201.

The ratio of wafer-to-ceramic heat transfer through direct conductionversus gap conduction is a function of the surface 202 characteristicsof the chuck 103 and the pressure and type of gas present in the gaps201. Heat transfer via gap conduction is expected to appreciablyinfluence the overall heat transfer between the wafer 105 and theceramic layer 201 when gas is present in the gaps 201. It should benoted that the amount of heat transfer via gap conduction is a functionof the portion of total chuck surface area underlying the wafer that isin contact with the wafer, physical characteristics of the gaps, type ofgas present within the gaps, and pressure of gas present within thegaps. The amount of gaps and physical properties of the gaps, i.e., sizeand shape of the gaps, can be characterized by a surface roughnessparameter and/or the fraction of the area in contact with the wafer.

It will be appreciated by those skilled in the art that the surfaceroughness parameter can be represented as a root-mean-square (RMS)surface roughness measurement. In one embodiment, surface 202 of thechuck 103 is defined to have a RMS surface roughness within a rangeextending from about 5 microinches to about 200 microinches. In a morepreferred embodiment, surface 202 of the chuck 103 is defined to have aRMS surface roughness within a range extending from about 20 microinchesto about 100 microinches. In an alternative embodiment, the surface 202of the chuck 103 can be defined to include engineered gaps. Theengineering gaps can be configured to enhance the influence of gapconductance on the overall heat transfer between the wafer 105 and theceramic layer 203.

Referring back to the chuck 103 of FIG. 2, heat is conducted from theceramic layer 203 to the adhesive layer 205. From the adhesive layer205, heat is conducted to the aluminum layer 206. Heat is then conductedfrom the aluminum layer 206 to the fluid within the coolant channels207. In a preferred embodiment, the fluid within the coolant channels207 effectively performs as a perfect heat sink. It should beappreciated that the fluid type, fluid temperature, and fluid flow ratewithin the coolant channels 207 can be defined to approach theperformance of the perfect heat sink.

Many parameters that influence heat transfer characteristics of thechuck 103 are invariable with time and use. For example, a thickness ofthe adhesive layer 205 and a mass of the aluminum layer 206 are notexpected to change with time. However, it should be understood that theas-fabricated values for such invariable heat transfer characteristicsof the chuck 103 can differ from one chuck to another. Therefore,different chucks may have different initial thermal characteristics.Additionally, the manner in which different chucks are installed canaffect the thermal performance of the chuck during plasma processing.Furthermore, it should be appreciated that some chuck properties aresubject to change as a function of use. As a consequence, the thermalperformance of the chuck may change as a function of use, and cause acorresponding change in the wafer temperature. Due to the directinfluence of the chuck thermal performance on the wafer temperature, itis important to understand and control the chuck thermal performanceduring wafer processing.

As previously discussed, one property of the chuck that is subject tochange and that can significantly affect the wafer temperature is thesurface roughness of the chuck surface interfacing with the wafer(“interfacing surface”). During use, the interfacing surface roughnesscan be modified by various processes, such as a waferless auto cleanprocess. Additionally, with extended use, the wafer itself can modifythe interfacing surface roughness through physical wear. To maintain aconsistent thermal interface between the wafer and the chuck, it isnecessary to quantify the interfacing surface roughness and anymodification thereof.

The present invention provides a method for quantifying the thermalcharacteristics of the chuck 103, including the wafer-to-chuckinterface, in order to maintain a target wafer temperature during plasmaprocessing. In one embodiment, a transient response of the wafertemperature to a change in plasma power is measured for differentbackside gas pressures, wherein the backside gas pressure corresponds tothe gas pressure within the gaps 201 at the interface between the wafer105 and the chuck 103. FIG. 3 is an illustration showing transient wafertemperature measurements for a chuck designated “ESC 1” at variousbackside gas pressures, in accordance with one embodiment of the presentinvention. Measurement of the wafer temperature transient response ateach backside gas pressure begins by measuring the wafer temperature inthe presence of a stead-state plasma 107. With respect to FIG. 3, thepresence of the steady-state plasma is designated by “Plasma On.” Then,the plasma is turned off and the temperature of the wafer is measured asa function of cooling time for each gas pressure (301-309). With respectto FIG. 3, the absence of the plasma is designated by “Plasma Off.”

When the plasma is present, the wafer temperature is essentially atsteady state. When the plasma is turned off, the wafer temperature dropsaccording the heat transfer characteristics of the chuck 103. It shouldbe appreciated that when evaluating the thermal performance of the chuck103 in the absence of the plasma, it is not necessary to consider a heatflux contribution from the plasma to the wafer. Therefore, the transientbehavior of the wafer temperature after the plasma is turned off can bedirectly attributed to the chuck 103 thermal characteristics. Once thewafer temperature measurements are obtained, a curve fit can be made tothe temperature versus time data for each backside gas pressure. Withrespect to FIG. 3, five backside gas pressures are evaluated asrepresented by curves 301-309. The wafer temperature versus time curvesare analyzed to determine a thermal characterization parameter value forthe chuck 103. In one embodiment, the thermal characterization parameterfor the chuck 103 is represented as an effective thermal conductivity ofthe chuck 103. In another embodiment, the thermal characterizationparameter for the chuck 103 is represented as a derived parameter, e.g.,time constant, that characterizes the thermal performance of the chuck103. The chuck thermal characterization parameter value determined foreach of the temperature versus time curves corresponds to the associatedbackside gas pressure. It should be appreciated that the transient wafertemperature response to plasma power shut off for different backside gaspressures is determined for each chuck of interest.

The measured thermal characterization parameter values for the variousbackside gas pressures, as determined from the transient wafertemperature response curves such as those shown in FIG. 3, are used togenerate a thermal characterization parameter versus backside gaspressure curve for the corresponding chuck. FIG. 4 is an illustrationshowing exemplary thermal characterization parameter versus backside gaspressure curves, in accordance with one embodiment of the presentinvention. For ease of discussion, the thermal characterizationparameter versus backside gas pressure curve for a given chuck isreferred to as a “thermal calibration curve.” FIG. 4 shows thermalcalibration curves 401 and 405 for each of two chucks designated “ESC 1”and “ESC 2”, respectively. It should be appreciated that the differencein slope between the thermal calibration curves of ESC 1 and ESC 2 isdue to differences in the wafer interfacing surface roughness of eachchuck. For example, the chuck having a surface roughness that allows formore gap exposure at the backside of the wafer can be expected toexhibit a stronger influence of backside gas pressure on wafertemperature.

The thermal calibration curves for each chuck can be used to determine achuck-specific backside gas pressure corresponding to a particulartarget thermal parameter value, wherein the target thermal parametervalue represents a target chuck thermal performance. It should beappreciated that different chucks having their backside gas pressurestuned to the same target thermal parameter value will have asubstantially similar overall heat transfer capability. Thus, wafertemperatures associated with the different chucks tuned to the sametarget thermal parameter value will be substantially similar. Therefore,the thermal calibration curves for various chucks can be used to tunethe various chucks, via backside gas pressure, in order to match wafertemperatures during plasma processing. The required backside gaspressure for a particular chuck to obtain a target heat transfercapability can be specified explicitly or as a pressure offset relativeto an initial backside gas pressure specified for the chuck. In oneembodiment, the backside gas is helium and is controllable within apressure range extending from about 5 ton to about 100 ton.

In another embodiment, the chuck thermal performance can be calibratedwith respect to multiple thermal characterization parameters. Forexample, the backside gas pressure can be considered in combination withone or more additional parameters, e.g., chuck chiller temperature orchuck heater temperature, when calibrating the thermal performance ofthe chuck. In this embodiment, different chucks having the combinationof backside gas pressure and additional parameter(s) tuned to achievethe same target thermal performance will have a substantially similaroverall heat transfer capability, resulting in substantially similarwafer temperatures. Therefore, the present invention also provides foruse of multiple-parameter thermal calibration data to tune variouschucks to match wafer temperatures during plasma processing.

FIG. 5 is an illustration showing a flowchart of a method for performinga thermal calibration of a chuck, in accordance with one embodiment ofthe present invention. The method includes an operation 501 forsupporting a wafer (or equivalent test object) on the chuck in exposureto a heat source. The method continues with an operation 503 forapplying a gas at a substantially constant pressure at an interfacebetween the wafer and the chuck. In an operation 505, the heat source isremoved. Then, an operation 507 is performed to measure a temperature ofthe wafer as a function of time while maintaining the applied gaspressure. During operations 501-507, control parameters of the chuckother than the applied gas pressure are set to values to be appliedduring use of the chuck in a wafer fabrication process.

The method further includes an operation 509 for determining a thermalcharacterization parameter value based on the measured temperature ofthe wafer as a function of time. In one embodiment, the thermalcharacterization parameter is defined as a time constant within amathematical model representing the measured temperature of the wafer asa function of time. In another embodiment, the thermal characterizationparameter is defined as an effective thermal conductivity value of thechuck. Regardless of the particular thermal characterization parameterembodiment, it should be appreciated that the determined thermalcharacterization parameter values are directly correlated to the appliedgas pressure. In an operation 511, the operations 501-509 are repeatedfor a number of different applied gas pressures. In one embodiment, thenumber of different applied gas pressures are within a range extendingfrom about 5 ton to about 100 ton. Following the operation 511, a set ofmeasured thermal characterization parameter data is obtained for anumber of applied gas pressures. In an operation 513, a correlation isestablished between the measured thermal characterization parameter dataand the corresponding gas pressures to generate a thermal calibrationcurve for the chuck.

It should be appreciated that using the generated thermal calibrationcurve, a gas pressure can be determined for a particular thermalcharacterization parameter value, vice-versa. Additionally, the thermalcharacterization parameter represented in the thermal calibration curvefor the chuck can be directly correlated to wafer temperatures duringthe wafer fabrication process. Thus, the thermal calibration curve forthe chuck can be represented in an alternative format as in-processwafer temperature versus applied gas pressure.

In an alternate embodiment, the operations 501 through 511 in the methodof FIG. 5 are repeated for a number of different values of an additionalthermal influencing parameter to be considered in combination with theapplied gas pressures. For example, the additional thermal influencingparameter may be a chuck chiller temperature or a chuck heatertemperature. In this alternate embodiment, the previously discussedoperation 513 is slightly modified to establish a correlation betweenthe determined chuck thermal characterization parameter values andcombinations of gas pressure values and additional thermal influencingparameter values to generate a multiple-parameter thermal calibrationcurve for the chuck. The multiple-parameter thermal calibration curvecan be used to tune the chuck to achieve a particular wafer temperatureduring plasma processing.

FIG. 6 is an illustration showing a flowchart of a method forcontrolling wafer temperature during a wafer fabrication process, inaccordance with one embodiment of the present invention. In an operation601, a chuck target thermal characterization parameter value isdetermined. It should be appreciated that the chuck is to be used tohold the wafer during the wafer fabrication process. The chuck targetthermal characterization parameter value corresponds to a target thermalperformance of the chuck which results in a target wafer temperatureduring the wafer fabrication process. In one embodiment, the chucktarget thermal characterization parameter value is defined as a targeteffective thermal conductivity value of the chuck. In anotherembodiment, the chuck target thermal characterization parameter value isdefined as a target time constant value for heat transfer through thechuck.

The method further includes an operation 603 for examining a thermalcalibration curve for the chuck to determine a backside gas pressurevalue corresponding to the chuck target thermal characterizationparameter. It should be understood that the backside gas pressurecorresponds to a pressure of a gas applied at an interface between thewafer and the chuck. In one embodiment, the gas applied at the interfacebetween the wafer and the chuck is helium. In an operation 605, thebackside gas pressure is set to the value corresponding to the chucktarget thermal characterization parameter value. Following the operation605, the fabrication process is performed on the wafer while maintainingthe backside gas pressure at the value corresponding to the chuck targetthermal characterization parameter value. It should be appreciated thatthe backside gas pressure serves to control the wafer temperature duringthe fabrication process.

With respect to initial variability in thermal characteristics amongchucks, the method of the present invention for developing and usingchuck-specific thermal calibration curves will enable chucks inas-fabricated condition to be tuned to a target heat transfercapability. For example, thermal calibration curves can be developed foreach chuck based on its as-fabricated state. Then, the backside gaspressure of each chuck during operation can be set to match a targetvalue for a thermal characterization parameter. Matching of each chuckto the target thermal characterization parameter value will cause eachchuck to conduct heat away from the wafer at essentially the same rate.Thus, the wafer temperatures corresponding to the various tuned chuckswill be essentially the same.

As the chuck is used over time, the thermal calibration curve for thechuck can be updated to reflect the latest condition of the chuck. Thus,the updated thermal calibration curve will capture chuck propertychanges that affect thermal performance, such as surface roughness atthe wafer-to-chuck interface. It should be appreciated that an updatefrequency for the thermal calibration curve associated with a particularchuck will be dependent upon how well the thermal calibration curvecontinues to predict the thermal performance of the chuck. For example,when the backside gas pressure specified by an existing thermalcalibration curve begins to yield a different chuck thermal performance,as indicated by measured wafer temperatures, it will be necessary torepeat the thermal calibration curve development process to obtain anupdated thermal calibration curve that reflects the latest physical andthermal condition of the chuck.

In wafer fabrication, it is often desirable to run the same waferprocess in multiple chambers to get the same resulting wafer condition.If the wafer temperature in each chamber differs due to chuck thermalcharacteristics, it will be difficult if not impossible to obtain thesame resulting wafer condition from each chamber. With the presentinvention, the chuck in each chamber can be tuned, via backside gaspressure as specified by the corresponding thermal calibration curve, toprovide a consistent thermal performance. Thus, the tuned chucks willenable consistent wafer temperatures to be maintained in each chamberduring wafer processing. Consequently, the post-process wafer conditionassociated with each chamber will not exhibit substantial variation aswould be expected with in-process wafer temperature variations amongchambers.

The present invention can also be embodied as a system for providingwafer temperature control during a wafer fabrication process. The systemincludes a chuck, a gas controller, and a computing device forcontrolling the gas controller. The chuck is defined to hold a wafer inexposure to a plasma during a fabrication process. The chuck includes anumber of ports to supply a gas at an interface between the wafer andthe chuck. The gas controller is defined to control a pressure of thegas supplied at the interface between the wafer and the chuck. Thecomputing device includes thermal calibration data for the chuck. Itshould be appreciated that in various embodiments the thermalcalibration data can be maintained in different formats, such as a tableof values or parameters for an equation defined to fit the data. Thethermal calibration data specifies a gas pressure setting to becommunicated from the computing device to the gas controller to maintaina target wafer temperature. The target wafer temperature is correlatedto a particular chuck target thermal characterization parameter valuerepresented by the thermal calibration data. The thermal calibrationdata for the chuck represents the chuck thermal characterizationparameter as a function of the pressure of the gas supplied at theinterface between the wafer and the chuck. In one embodiment, thethermal characterization parameter for the chuck is defined as aneffective thermal conductivity of the chuck. In another embodiment, thethermal characterization parameter for the chuck is defined as a timeconstant for heat transfer through the chuck.

It should be appreciated that development and use of the chuck thermalcalibration curves enable pre-conditioning of the chuck to limit processdrift. Additionally, the chuck thermal calibration curves enablecompensation for variation in chuck thermal characteristics as afunction of operation time. Furthermore, the chuck thermal calibrationcurves of the present invention allow the wafer temperature to becontrolled using the chuck rather than other non-chuck related processparameters such as RF power, chamber pressure, etc. Due to dependenciesbetween the non-chuck related process parameters, adjustment of thenon-chuck related process parameters in an attempt to control wafertemperature can lead to a narrowing of a process window, wherein theprocess window is defined by an acceptable range of each processparameter capable of affecting the wafer results. Because of the chuck'sindependence from the process window, the wafer temperature can becontrolled by adjusting the overall thermal performance of the chuckwithout narrowing the process window.

While this invention has been described in terms of several embodiments,it will be appreciated that those skilled in the art upon reading thepreceding specifications and studying the drawings will realize variousalterations, additions, pet mutations and equivalents thereof.Therefore, it is intended that the present invention includes all suchalterations, additions, permutations, and equivalents as fall within thetrue spirit and scope of the invention.

1. A method for performing a thermal calibration of a chuck, comprising:(a) supporting a wafer on the chuck in exposure to a heat source; (b)applying a gas at a substantially constant pressure at an interfacebetween the wafer and the chuck; (c) removing the heat source; (d)measuring a temperature of the wafer as a function of time followingremoval of the heat source while maintaining the applied gas pressure;(e) determining a chuck thermal characterization parameter value basedon the measured temperature of the wafer as a function of time, whereinthe determined chuck thermal characterization parameter valuecorresponds to the applied gas pressure; (f) repeating operations (a)through (e) for a number of different applied gas pressures; and (g)establishing a correlation between the determined chuck thermalcharacterization parameter values and corresponding gas pressures togenerate a thermal calibration curve for the chuck.
 2. The method ofclaim 1, further comprising: setting control parameters of the chuckother than the applied gas pressure to values to be applied during useof the chuck in a wafer fabrication process.
 3. The method of claim 1,further comprising: determining a gas pressure required to obtain aparticular chuck thermal characterization parameter value according tothe thermal calibration curve for the chuck, wherein the particularchuck thermal characterization parameter value corresponds to targetthermal performance of the chuck.
 4. The method of claim 3, wherein thetarget thermal performance of the chuck corresponds to a target wafertemperature during a wafer fabrication process.
 5. The method of claim1, wherein the heat source is a plasma maintained at a substantiallyconstant power level.
 6. The method of claim 1, further comprising:repeating operations (a) through (f) for a number of different values ofan additional thermal influencing parameter to be considered incombination with the applied gas pressures; and establishing acorrelation between the determined chuck thermal characterizationparameter values and combinations of gas pressure values and additionalthermal influencing parameter values to generate a multiple-parameterthermal calibration curve for the chuck.
 7. The method of claim 1,wherein the chuck thermal characterization parameter value is defined asa time constant value within a mathematical model representing themeasured temperature of the wafer as a function of time.
 8. The methodof claim 1, wherein the chuck thermal characterization parameter valueis defined as an effective thermal conductivity value of the chuck. 9.The method of claim 1, wherein the number of different applied gaspressures are within a range extending from about 5 ton to about 100torr.
 10. A thermally calibrated chuck for supporting a semiconductorwafer during a fabrication process, comprising: a first material layerhaving an upper surface upon which a wafer is to be supported, whereinthe upper surface includes portions that physically contact the waferand portions that form gaps between the upper surface and the wafer whenpresent; a second material layer defined to support the first materiallayer, the second material layer formed of a thermally conductivematerial and including a number of coolant channels; and gas coolantchannels defined to direct a gas coolant to portions of the uppersurface that form gaps between the upper surface and the wafer whenpresent, wherein the chuck is characterized by a thermal calibrationcurve that represents a thermal interface between the upper surface andthe wafer when present, heat transfer through the first material layerto the second material layer, and heat transfer through the secondmaterial layer to the number of coolant channels.
 11. The thermallycalibrated chuck for supporting a semiconductor wafer during afabrication process as recited in claim 10, further comprising: anadhesive layer defined to bind the first material layer to the secondmaterial layer.
 12. The thermally calibrated chuck for supporting asemiconductor wafer during a fabrication process as recited in claim 10,wherein the first material layer is formed of a ceramic material, andwherein the second material layer is formed of aluminum.
 13. Thethermally calibrated chuck for supporting a semiconductor wafer during afabrication process as recited in claim 10, wherein the portions of theupper surface that form gaps between the upper surface and the waferwhen present are defined by a surface roughness of the upper surface.14. The thermally calibrated chuck for supporting a semiconductor waferduring a fabrication process as recited in claim 13, wherein the surfaceroughness of the upper surface is defined by a root-mean-square surfaceroughness within a range extending from 20 microinches to 100microinches.
 15. The thermally calibrated chuck for supporting asemiconductor wafer during a fabrication process as recited in claim 10,wherein the portions of the upper surface that form gaps between theupper surface and the wafer when present are defined by an engineeredsurface topography.
 16. The thermally calibrated chuck for supporting asemiconductor wafer during a fabrication process as recited in claim 15,wherein the engineered surface topography is defined as bumps on theupper surface.
 17. The thermally calibrated chuck for supporting asemiconductor wafer during a fabrication process as recited in claim 15,wherein the engineered surface topography includes engineered gapsbetween the upper surface and the wafer when present.
 18. The thermallycalibrated chuck for supporting a semiconductor wafer during afabrication process as recited in claim 10, wherein the gas coolantchannels are defined to apply a pressure of the gas coolant to control athermal interface between the upper surface and the wafer when present.19. The thermally calibrated chuck for supporting a semiconductor waferduring a fabrication process as recited in claim 18, wherein the thermalcalibration curve corresponds to measured chuck thermal characterizationparameter values as a function of the pressure of the gas coolant. 20.The thermally calibrated chuck for supporting a semiconductor waferduring a fabrication process as recited in claim 19, wherein themeasured chuck thermal characterization parameter value for a given gascoolant pressure is based on a measured temperature decline of the waferas a function of time following removal of a heat source above the waferwith the given gas coolant pressure applied within the gaps between theupper surface and the wafer.